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  15. <title>GRDTREND</title>
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  18. <body bgcolor="#ffffff">
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  19. 

  20. <h1 align=center>GRDTREND</h1>
    21. 

  21. 

  22. <a href="#NAME">NAME</a><br>
    23. 

  23. <a href="#SYNOPSIS">SYNOPSIS</a><br>
    24. 

  24. <a href="#DESCRIPTION">DESCRIPTION</a><br>
    25. 

  25. <a href="#OPTIONS">OPTIONS</a><br>
    26. 

  26. <a href="#REMARKS">REMARKS</a><br>
    27. 

  27. <a href="#GRID FILE FORMATS">GRID FILE FORMATS</a><br>
    28. 

  28. <a href="#EXAMPLES">EXAMPLES</a><br>
    29. 

  29. <a href="#SEE ALSO">SEE ALSO</a><br>
    30. 

  30. 

  31. <hr>
    32. 

  32. 

  33. 

  34. <a name="NAME"></a>
    35. 

  35. <h2>NAME</h2>
    36. 

  36. 

  37. 

  38. <p style="margin-left:11%; margin-top: 1em">grdtrend
    39. 

  39. &minus; Fit and/or remove a polynomial trend in a grid
    40. 

  40. file</p>
    41. 

  41. 

  42. <a name="SYNOPSIS"></a>
    43. 

  43. <h2>SYNOPSIS</h2>
    44. 

  44. 

  45. 

  46. 

  47. <p style="margin-left:11%; margin-top: 1em"><b>grdtrend</b>
    48. 

  48. <i>grdfile</i> <b>&minus;N</b><i>n_model</i>[<b>r</b>] [
    49. 

  49. <b>&minus;D</b><i>diff.grd</i> ] [
    50. 

  50. <b>&minus;T</b><i>trend.grd</i> ] [ <b>&minus;V</b> ] [
    51. 

  51. <b>&minus;W</b><i>weight.grd</i> ]</p>
    52. 

  52. 

  53. <a name="DESCRIPTION"></a>
    54. 

  54. <h2>DESCRIPTION</h2>
    55. 

  55. 

  56. 

  57. 

  58. <p style="margin-left:11%; margin-top: 1em"><b>grdtrend</b>
    59. 

  59. reads a 2-D grid file and fits a low-order polynomial trend
    60. 

  60. to these data by [optionally weighted] least-squares. The
    61. 

  61. trend surface is defined by:</p>
    62. 

  62. 

  63. <p style="margin-left:11%; margin-top: 1em">m1 + m2*x +
    64. 

  64. m3*y + m4*x*y + m5*x*x + m6*y*y + m7*x*x*x + m8*x*x*y +
    65. 

  65. m9*x*y*y + m10*y*y*y.</p>
    66. 

  66. 

  67. <p style="margin-left:11%; margin-top: 1em">The user must
    68. 

  68. specify <b>&minus;N</b><i>n_model</i>, the number of model
    69. 

  69. parameters to use; thus, <b>&minus;N</b><i>4</i> fits a
    70. 

  70. bilinear trend, <b>&minus;N</b><i>6</i> a quadratic surface,
    71. 

  71. and so on. Optionally, append <b>r</b> to the
    72. 

  72. <b>&minus;N</b> option to perform a robust fit. In this
    73. 

  73. case, the program will iteratively reweight the data based
    74. 

  74. on a robust scale estimate, in order to converge to a
    75. 

  75. solution insensitive to outliers. This may be handy when
    76. 

  76. separating a &quot;regional&quot; field from a
    77. 

  77. &quot;residual&quot; which should have non-zero mean, such
    78. 

  78. as a local mountain on a regional surface.</p>
    79. 

  79. 

  80. <p style="margin-left:11%; margin-top: 1em">If data file
    81. 

  81. has values set to NaN, these will be ignored during fitting;
    82. 

  82. if output files are written, these will also have NaN in the
    83. 

  83. same locations.</p>
    84. 

  84. 

  85. <p style="margin-left:11%; margin-top: 1em">No space
    86. 

  86. between the option flag and the associated arguments.
    87. 

  87. <i><br>
    88. 

  88. grdfile</i></p>
    89. 

  89. 

  90. <p style="margin-left:22%;">The name of a 2-D binary grid
    91. 

  91. file.</p>
    92. 

  92. 

  93. <table width="100%" border=0 rules="none" frame="void"
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  94.        cellspacing="0" cellpadding="0">
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  95. <tr valign="top" align="left">
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  96. <td width="11%"></td>
    97. 

  97. <td width="3%">
    98. 

  98. 

  99. 

  100. 

  101. <p style="margin-top: 1em" valign="top"><b>&minus;N</b></p> </td>
    102. 

  102. <td width="8%"></td>
    103. 

  103. <td width="78%">
    104. 

  104. 

  105. 

  106. 

  107. <p style="margin-top: 1em" valign="top">[<b>r</b>]<i>n_model</i>
    108. 

  108. sets the number of model parameters to fit. Append <b>r</b>
    109. 

  109. for robust fit.</p></td>
    110. 

  110. </table>
    111. 

  111. 

  112. <a name="OPTIONS"></a>
    113. 

  113. <h2>OPTIONS</h2>
    114. 

  114. 

  115. 

  116. <p style="margin-left:11%; margin-top: 1em">No space
    117. 

  117. between the option flag and the associated arguments.</p>
    118. 

  118. 

  119. <table width="100%" border=0 rules="none" frame="void"
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  120.        cellspacing="0" cellpadding="0">
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  121. <tr valign="top" align="left">
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  122. <td width="11%"></td>
    123. 

  123. <td width="3%">
    124. 

  124. 

  125. 

  126. 

  127. <p style="margin-top: 1em" valign="top"><b>&minus;D</b></p> </td>
    128. 

  128. <td width="8%"></td>
    129. 

  129. <td width="78%">
    130. 

  130. 

  131. 

  132. <p style="margin-top: 1em" valign="top">Write the
    133. 

  133. difference (input data - trend) to the file
    134. 

  134. <i>diff.grd</i>.</p> </td>
    135. 

  135. <tr valign="top" align="left">
    136. 

  136. <td width="11%"></td>
    137. 

  137. <td width="3%">
    138. 

  138. 

  139. 

  140. 

  141. <p style="margin-top: 1em" valign="top"><b>&minus;T</b></p> </td>
    142. 

  142. <td width="8%"></td>
    143. 

  143. <td width="78%">
    144. 

  144. 

  145. 

  146. <p style="margin-top: 1em" valign="top">Write the fitted
    147. 

  147. trend to the file <i>trend.grd</i>.</p></td>
    148. 

  148. <tr valign="top" align="left">
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  149. <td width="11%"></td>
    150. 

  150. <td width="3%">
    151. 

  151. 

  152. 

  153. 

  154. <p style="margin-top: 1em" valign="top"><b>&minus;V</b></p> </td>
    155. 

  155. <td width="8%"></td>
    156. 

  156. <td width="78%">
    157. 

  157. 

  158. 

  159. <p style="margin-top: 1em" valign="top">Selects verbose
    160. 

  160. mode, which will send progress reports to stderr [Default
    161. 

  161. runs &quot;silently&quot;].</p></td>
    162. 

  162. <tr valign="top" align="left">
    163. 

  163. <td width="11%"></td>
    164. 

  164. <td width="3%">
    165. 

  165. 

  166. 

  167. 

  168. <p style="margin-top: 1em" valign="top"><b>&minus;W</b></p> </td>
    169. 

  169. <td width="8%"></td>
    170. 

  170. <td width="78%">
    171. 

  171. 

  172. 

  173. <p style="margin-top: 1em" valign="top">If
    174. 

  174. <i>weight.grd</i> exists, it will be read and used to solve
    175. 

  175. a weighted least-squares problem. [Default: Ordinary
    176. 

  176. least-squares fit.] If the robust option has been selected,
    177. 

  177. the weights used in the robust fit will be written to
    178. 

  178. <i>weight.grd</i>.</p> </td>
    179. 

  179. </table>
    180. 

  180. 

  181. <a name="REMARKS"></a>
    182. 

  182. <h2>REMARKS</h2>
    183. 

  183. 

  184. 

  185. <p style="margin-left:11%; margin-top: 1em">The domain of x
    186. 

  186. and y will be shifted and scaled to [-1, 1] and the basis
    187. 

  187. functions are built from Legendre polynomials. These have a
    188. 

  188. numerical advantage in the form of the matrix which must be
    189. 

  189. inverted and allow more accurate solutions. NOTE: The model
    190. 

  190. parameters listed with <b>&minus;V</b> are Legendre
    191. 

  191. polynomial coefficients; they are not numerically equivalent
    192. 

  192. to the m#s in the equation described above. The description
    193. 

  193. above is to allow the user to match <b>&minus;N</b> with the
    194. 

  194. order of the polynomial surface. See <b><A HREF="grdmath.html">grdmath</A></b> if you
    195. 

  195. need to evaluate the trend using the reported
    196. 

  196. coefficients.</p>
    197. 

  197. 

  198. <a name="GRID FILE FORMATS"></a>
    199. 

  199. <h2>GRID FILE FORMATS</h2>
    200. 

  200. 

  201. 

  202. <p style="margin-left:11%; margin-top: 1em">By default
    203. 

  203. <b><A HREF="GMT.html">GMT</A></b> writes out grid as single precision floats in a
    204. 

  204. COARDS-complaint netCDF file format. However, <b><A HREF="GMT.html">GMT</A></b> is
    205. 

  205. able to produce grid files in many other commonly used grid
    206. 

  206. file formats and also facilitates so called
    207. 

  207. &quot;packing&quot; of grids, writing out floating point
    208. 

  208. data as 2- or 4-byte integers. To specify the precision,
    209. 

  209. scale and offset, the user should add the suffix
    210. 

  210. <b>=</b><i>id</i>[<b>/</b><i>scale</i><b>/</b><i>offset</i>[<b>/</b><i>nan</i>]],
    211. 

  211. where <i>id</i> is a two-letter identifier of the grid type
    212. 

  212. and precision, and <i>scale</i> and <i>offset</i> are
    213. 

  213. optional scale factor and offset to be applied to all grid
    214. 

  214. values, and <i>nan</i> is the value used to indicate missing
    215. 

  215. data. When reading grids, the format is generally
    216. 

  216. automatically recognized. If not, the same suffix can be
    217. 

  217. added to input grid file names. See <b><A HREF="grdreformat.html">grdreformat</A></b>(1)
    218. 

  218. and Section 4.17 of the GMT Technical Reference and Cookbook
    219. 

  219. for more information.</p>
    220. 

  220. 

  221. <p style="margin-left:11%; margin-top: 1em">When reading a
    222. 

  222. netCDF file that contains multiple grids, <b><A HREF="GMT.html">GMT</A></b> will
    223. 

  223. read, by default, the first 2-dimensional grid that can find
    224. 

  224. in that file. To coax <b><A HREF="GMT.html">GMT</A></b> into reading another
    225. 

  225. multi-dimensional variable in the grid file, append
    226. 

  226. <b>?</b><i>varname</i> to the file name, where
    227. 

  227. <i>varname</i> is the name of the variable. Note that you
    228. 

  228. may need to escape the special meaning of <b>?</b> in your
    229. 

  229. shell program by putting a backslash in front of it, or by
    230. 

  230. placing the filename and suffix between quotes or double
    231. 

  231. quotes. The <b>?</b><i>varname</i> suffix can also be used
    232. 

  232. for output grids to specify a variable name different from
    233. 

  233. the default: &quot;z&quot;. See <b><A HREF="grdreformat.html">grdreformat</A></b>(1) and
    234. 

  234. Section 4.18 of the GMT Technical Reference and Cookbook for
    235. 

  235. more information, particularly on how to read splices of 3-,
    236. 

  236. 4-, or 5-dimensional grids.</p>
    237. 

  237. 

  238. <a name="EXAMPLES"></a>
    239. 

  239. <h2>EXAMPLES</h2>
    240. 

  240. 

  241. 

  242. <p style="margin-left:11%; margin-top: 1em">To remove a
    243. 

  243. planar trend from hawaii_topo.grd and write result in
    244. 

  244. hawaii_residual.grd:</p>
    245. 

  245. 

  246. 

  247. <p style="margin-left:11%; margin-top: 1em"><b>grdtrend</b>
    248. 

  248. hawaii_topo.grd <b>&minus;N</b>3
    249. 

  249. <b>&minus;D</b>hawaii_residual.grd</p>
    250. 

  250. 

  251. <p style="margin-left:11%; margin-top: 1em">To do a robust
    252. 

  252. fit of a bicubic surface to hawaii_topo.grd, writing the
    253. 

  253. result in hawaii_trend.grd and the weights used in
    254. 

  254. hawaii_weight.grd, and reporting the progress:</p>
    255. 

  255. 

  256. 

  257. <p style="margin-left:11%; margin-top: 1em"><b>grdtrend</b>
    258. 

  258. hawaii_topo.grd <b>&minus;N</b>10<b>r
    259. 

  259. &minus;T</b>hawaii_trend.grd
    260. 

  260. <b>&minus;W</b>hawaii_weight.grd <b>&minus;V</b></p>
    261. 

  261. 

  262. <a name="SEE ALSO"></a>
    263. 

  263. <h2>SEE ALSO</h2>
    264. 

  264. 

  265. 

  266. <p style="margin-left:11%; margin-top: 1em"><i><A HREF="GMT.html">GMT</A></i>(1),
    267. 

  267. <i><A HREF="grdfft.html">grdfft</A></i>(1), <i><A HREF="grdfilter.html">grdfilter</A></i>(1)</p>
    268. 

  268. <hr>
    269. 

  269. </body>
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  270. </html>
    271. 

  271.